The present invention relates to electroconductive particles, in particular gold-coated polymer microbeads, and to a process for their preparation.
The use of electroconductive particles to achieve an anisotropic electrical connection between electronic components has been described in EP-A-265212 (Japan Synthetic Rubber) and in JP-B-5/19241 (Sumitomo Chemical). Metal coated polymer beads, distributed in an adhesive matrix are disposed between the component surfaces and serve to provide an electrical connection between those surfaces.
In JP-B-5/19241, gold, silver, copper, nickel and aluminum are said to be suitable electroconductive coatings and the polymer is stated to be a styrene copolymer, e.g. with an acrylic acid ester, a methacrylic acid ester, an unsaturated carboxylic acid, a nitrile or a diene. The copolymer must however contain styrene as its principal component and no more than 30 wt. % of the polymer can derive from another ethylenically unsaturated monomer. In the Examples a divinyl benzene:styrene (4:96 wt. ratio) polymer particle is either acid treated, treated with colloidal palladium chloride and then plated with silver or is coated with silver by vacuum vapour deposition.
EP-A-265212 describes the use of polystyrene, styrene/divinylbenzene/methacrylic acid (65:35:5 by weight), methyl methacrylate/divinylbenzene (50:50), and styrene/divinyl benzene/sodium styrene sulphonate (65:30:5) polymer particles. The surfaces of the particles were chemically etched, treated with colloidal palladium chloride and then plated with nickel or copper. Subsequent gold plating of such nickel plated styrene/divinyl benzene/sodium styrene sulphonate particles was also described and the resulting particles displayed the best results in terms of anisotropic conductivity. Indeed Sumitomo refer to gold plating as being especially preferred for obtaining a good stable and long lasting electrical connection.
The electroconductive particles are substantially spherical having an average particle diameter in the micrometer range, e.g. 1 to 50 xcexcm. Gold plated polymer microspheres of this type are available commercially from Sekisui.
In application and subsequent use as electrical connectors, the electroconductive microparticles undergo mechanical and thermal stresses and for optimum performance the coatings should be substantially blemish free. Where inert gold coatings are used, to avoid contamination any underlying non-gold metal coating should not be exposed.
However when gold-plated polymer microspheres produced as described above are examined by electron microscopy, it may clearly be seen that many of the gold surfaces are blemished and incomplete.
We have now found that improved gold-plated styrene copolymer microspheres may be produced by using a styrene copolymer in which styrene is a minor component and/or which incorporates a functionalised comonomer and is palladinated by reaction with a solution of a palladium compound.
Thus viewed from one aspect the invention provides electroconductive particles comprising a styrene copolymer core and an external gold coating, wherein the copolymer core comprises less than 50 wt. % styrene monomers.
Viewed from a further aspect the invention also provides electroconductive particles comprising a styrene copolymer core and an external gold coating, wherein the copolymer core comprises a styrene copolymer produced using styrene and a functionalized comonomer and subsequently palladinated by reaction with a solution of a palladium compound. functionalized comonomer and subsequently palladinated by reaction with a solution of a palladium compound.
Preferably the polymer core in the particles of the invention comprises less than 50 wt. % styrene residues and comprises residues of a functionalized comonomer.
By xe2x80x9cfunctionalizedxe2x80x9d it is meant that the comonomer contains reactive chemical groups that are not transformed into a non-reactive form by the polymerization reaction. Typically such groups will be pendant from the polymer backbone of the resultant copolymer. Exemplary functional groups include amine, thiol, hydroxyl, amide, ester and oxyacid groups, e.g. carboxyl, sulphonic, phosphoric and phosphonic groups. Preferred functional groups for the ready-to-palladinate particles include groups capable of coordinating palladium, e.g. amino, carboxyl, hydroxyl and sulphonic groups, especially amino groups.
The functional groups in the palladium treated particles need not be the same as in the comonomer used in the polymerization reaction, ie. they may be modified after the copolymer is prepared, e.g. by reaction with a bifunctional agent which serves to introduce the desired functional group. By way of example this may be achieved by reaction of a styrene:divinylbenzene: glycidylmethylmethacrylate copolymer with an aminating agent such as ethylene diamine.
The styrene content of the styrene copolymer in the palladium-treated particles is preferably 0.5 to 98.5 wt %, e.g. 20 to 95 wt. %, more preferably 10 to 75 wt. %, e.g. 25 to 70 wt. %. Where the styrene content is below 50 wt. %, it is preferably 20 to 49 wt. %, e.g. 25 to 45 wt. %, particularly 30 to 45 wt. %.
Where the styrene polymer is crosslinked, the most commonly used cross-linking comonomer is divinylbenzene (DVB). As used, however, DVB generally contains a relatively high proportion (about 20, 35 or 45% wt) of the monoethylenically unsaturated, styrene analog ethylvinylbenzene (EVB), and the styrene content of the polymer may be considered to be the sum of the styrene and the EVB.
The comonomers used with styrene to produce the polymer cores of the particles of the invention may be any comonomers capable of copolymerization with styrene. These will generally be compounds which are mono- or multiply ethylenically unsaturated; examples include acrylic acid esters (e.g. methyl, ethyl, butyl, glycidyl, etc. acrylate), methacrylate acid esters (e.g. methyl, ethyl, butyl, glycidyl, etc. methacrylates), acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, xcex1-methyl styrene, vinyl toluene, vinyl chloride, vinyl acetate, vinyl propionate etc. Acrylic and methacrylic acids and especially esters, and mixtures of two or more thereof are preferred. Other multiply ethylenically unsaturated monomers may be used to achieve a degree of crosslinking, e.g. dienes such as butadiene, divinyl benzene, trimethylolpropane acrylate or polyethylene glycol diacrylate, especially divinyl benzene. The proportion of multiply ethylenically unsaturated monomer used will depend on the resilience required of the polymer cores in use but their residues will generally make up from 2 to 30 wt. %, more preferably 5 to 20 wt. %, especially about 10 wt. % of the polymer. The proportion of the functionalized comonomer (e.g. an acrylate or methacrylate ester) will preferably be such that their residues make up 5 to 99.5 wt. %, e.g. 10 to 75 wt. %, more preferably 15 to 70 wt. %, especially 55 to 65 wt. % (e.g. 5 to 60 wt. %, for example 10 to 50 wt %). Particularly satisfactory results have been obtained with a styrene content of 25-35 wt %, a DVB content of 7 to 13 wt % and a methacrylate content of 55 to 65 wt %, and metal-plated particles having a core of such a polymer form a further aspect of the invention.
Particularly suitable copolymers include styrene: divinyl benzene:glycidyl methacrylate and styrene: divinyl benzene:glycidyl methacrylate:butyl methacrylate which may subsequently be aminated by treatment with ethylene diamine.
The polymer particles are preferably substantially spherical and also substantially monodisperse.
Thus the particle size distributions have a coefficient of variation CV of 20% or less, preferably 12% or less, more preferably 5% or less, still more preferably 3% or less, e.g. 1.5 to 2.5%. CV is determined in percent by the formula (standard deviation/mean)xc3x97100 where mean is the mean particle diameter and standard deviation is the standard deviation in particle size. CV is normally calculated by fitting a monomodal distribution curve to the detected particle size distribution. Thus some particles below or above mode size may be discounted in the calculation which may for example be based on about 90%, or more preferably about 98%, of the total detectable particle number. CV is often determined on a Coulter LS 130 particle analyser.
Mode particle size for the polymer core in the particles of the invention is preferably 0.5 to 500 xcexcm, more preferably 1 to 250 xcexcm, still more preferably 1 to 50 xcexcm, yet more preferably 1.5 to 20 xcexcm, especially 2 to 12 xcexcm, more especially 3 to 10 xcexcm.
Such substantially spherical and substantially monodisperse polymer particles may be produced by the technique described in US-A-4336173, WO93/02112 (Sinvent), JP-A-86/215602, JP-A-86/215603 and JP-A-86/215604.
As may be seen from the attached Figures, the use of low styrene content styrene copolymers can result in gold-plated microspheres which have a more xe2x80x9cfurryxe2x80x9d surface than those produced with higher styrene contents. This surprising effect results in lesser agglomeration of the particles and may also enable a better electrical contact to be made when the microspheres are sandwiched between electrical component surfaces.
As may also be seen from the attached Figures, the use of styrene copolymers which carry functional groups and are palladinated using a solution (rather than colloid) of a palladium compound results in coatings with fewer defects.
Where xe2x80x9cfurryxe2x80x9d particles are desired, the functional monomer (e.g. methacrylate) content should generally be increased for smaller particle sizes, e.g. 40 wt. % for 7 to 10 xcexcm particles rising almost to 100 wt. % for particles below 4.5 xcexcm.
Gold coating of the styrene copolymer particles may be achieved as described by Sumitomo (supra) and Japan Synthetic Rubber (supra), the contents of which are hereby incorporated by reference. However gold coating is preferably achieved by a multistep process which forms a further aspect of the invention.
According to this aspect the invention provides a process for the production of gold-plated polymer particles, said process comprising:
(i) palladinating functional groups in functional group containing styrene copolymer particles by contacting said particles with a solution of a palladium compound;
(ii) metal plating the palladinated particles with a metal having a standard electrode potential (Excex8) less than that of gold (1.42 V), in the presence of a reducing agent; and
(iii) gold-plating the metal plated particles, advantageously to a gold thickness of 50 to 600 xc3x85, e.g. 100 to 400 xc3x85 or 50 to 200 xc3x85, preferably about 400 xc3x85.
Conveniently the metal used in step (ii) is one having a standard electrode potential of from xe2x88x921.0V to +1.0V, e.g. zinc, iron, nickel, tin, lead, copper, chromium or silver. Nickel is especially preferred and the discussion and Examples below focus on the use of nickel. (For standard electrode potentials see for example CRC Handbook of Chemistry and Physics, 70th Edition, 1989, pages D151 et seq).
In the process of the invention, palladination may be achieved with any soluble palladium compound generally using water or another protic solvent. One exemplary complex is ammonium tetrachloro palladate. Advantageously an activator, e.g. dimethylaminoborane (DMAB) is also used. The palladination step is conveniently effected at elevated temperature, e.g. 50 to 70xc2x0 C., especially about 60xc2x0 C., and generally may be affected for 10 to 60 minutes, e.g. about 20 minutes.
The palladinated particles may then be nickel plated, or plated with other substrate metal coatings, using conventional electroless plating techniques, e.g. using a solution of a nickel compound (e.g. nickel chloride, nickel sulphate, etc., preferably nickel chloride) and a reducing agent (e.g. DMAB, hydrazine or hypophosphite). Ammonia may be used to achieve a pH of 10.5 to 11.0, optimally 10.8, Generally this electroless plating may be effected at or near ambient temperature and may be effected for about 20 minutes to 1 hour, e.g. 30 minutes. The resulting metal (e.g. nickel) coating is advantageously at least 200 xc3x85 thick, preferably 300-1000 xc3x85, e.g. about 400 xc3x85 or more preferably about 600 xc3x85. In this metal plating step, it is especially preferable to use DMAB as the reducing agent since this gives particularly good metal coating of the polymer particles.
During the metal-plating of the palladinated particles, it is especially preferred to disperse in the plating solution a stabilizer, e.g. an non-ionic surfactant (e.g. a Pluronic), a polyelectrolyte, a polymer (e.g. a polyelectrolyte), a silane, silicon or most preferably an inorganic oxide particulate, more particularly silica. Finely divided silica, e.g. having particle sizes of 3 to 50 nm, more particularly 5 to 15 nm, are especially preferred. Examples of suitable silicas include Cabosil, Aerosil 972 and, especially Aerosil 200 (available from Degussa, a 7 nm particle size hydrophilic silica). Such silicas may conveniently be used at concentrations of 0.05 to 5 g/L, especially 0.1 to 2 g/L.
The subsequent gold-plating step may again be effected using conventional gold-plating techniques, e.g. using potassium gold cyanide. This plating is preferably effected at elevated temperature, e.g. 70 to 90xc2x0 C., especially 80xc2x0 C., and may be effected for example for 20 minutes to 1 hour, e.g. 30 minutes. A xe2x80x9cGold Bathxe2x80x9d Aureus from Shipley may conveniently be used.
As with the initial metal coating of palladinated particles, it is especially preferred to use a stabilizer in the gold plating solution. For this purpose, the same stabilizers as mentioned above may be used, especially silicas. Following gold coating, it is also preferred to treat the gold plated particles with a stabilizer to reduce any tendency to agglomerate. In this regard, the preferred stabilizers are thiol stabilizers, especially thiols with hydroxyl, carboxyl, silane or sulphonic acid functionalities, most especially C2-25, especially C3-20, more especially C5-12, e.g. C6, thiols with hydroxyl functionalities, e.g. thiododecanol or more especially thiohexanol. In such thiols, the carbon backbone is preferably linear and xcex1, xcfx89 substituted.
Especially preferably, the gold plated particles are treated with the thiol stabilizer, e.g. mercaptohexan-1-ol, for 10 to 100 minutes, e.g. 25 to 35 minutes from ambient temperature at about 60xc2x0 C. The amount of thiol used is conveniently 4xc3x9710xe2x88x927 to 4xc3x9710xe2x88x925 mole/g particles (on a dry solids basis), particularly 10xe2x88x926 to 10xe2x88x925 mole/g, more particularly 2.5 to 5xc3x9710xe2x88x926 mole/g.
The gold-plated particles may then be formulated together with an adhesive to produce an electroconductive adhesive. Adhesives as described in Japan Synthetic Rubber (supra), EP-B-413161 and U.S. Pat. No. 5,162,087 may be used, e.g. radiation-curable or thermosetting resins. Such electroconductive adhesive compositions form a further aspect of the present invention. Viewed from this aspect the invention provides an electroconductive adhesive composition comprising an adhesive matrix material with disposed therein electroconductive particles, characterised in that said particles comprise electroconductive particles according to the invention or produced by the process of the invention.
The adhesive compositions of the invention may be used to connect electronic components, as described in EP-A-265212, EP-B-413161 and U.S. Pat. No. 5,162,087 for example. For example, the adhesive composition may be cast into films which are then laid between the components to be electrically connected. Typically such films may be 5-35 xcexcm, preferably 5-20 xcexcm, more preferably 10-15 xcexcm, e.g. about 10 xcexcm, thick.
Thus viewed from a further aspect the invention provides the use of the adhesive composition of the invention for providing an electrical connection (e.g. an isotropic or more preferably anisotropic connection) between electronic components.
Alternatively, the gold plated particles may be provided with an external (external to the gold layer that is) coating of an inorganic or organic material, preferably an electrical insulator, e.g. as described in U.S. Pat. No. 5,162,087, In this way the use of the particles as anisotropic conductors is further facilitated. Such further coated particles and adhesive compositions containing them form a further aspect of the invention. Thus for example the gold-plated particles may be provided with a thin, frangible coating of an insulator polymer, preferably one having a thickness of a few nm, e.g. 5-20 nm. In this way, a larger number of particles can be added to the adhesive since in-plane conduction is prevented.
As mentioned above, the palladinated particles may if desired be metal-coated with other metals than nickel, e.g. silver, copper, zinc, tin, etc. and such metal-coated particles (which may if desired subsequently be gold-plated) form a further aspect of the invention.
The gold- or other metal-plated particles of the invention may be used for purposes other than providing electrical connections. Thus for example they may be used in almost any of the currently existing uses for gold and gold-plated microbeads, e.g. in medical diagnostic assays, in gene therapy. Likewise they may be used in antibacterial, antifungal, anticorrosion, and antistatic applications, in paints, in polymer products (e.g. sheets, plates, moulded articles, etc.), in electromagnetic field shielding, in flooring products, etc. The particles may be formulated for such use in a conventional manner, e.g. by mixing with polymer moulding powder, by addition to adhesive compositions, by mixing with a paint base, etc. Such products and uses form further aspects of the invention. Viewed from one such aspect the invention provides a formed polymer product (e.g. a moulded article, a sheet, etc.) characterised in that metal-plated (i.e. plated with gold, nickel or other metals) polymer particles according to the invention are dispersed in the polymer. Viewed from another such aspect the invention provides a coating composition (e.g. a paint) comprising a liquid carrier with dispersed therein metal-plated polymer particles according to the invention.
Patent and other publications referred to herein are hereby incorporated by reference.